Synthesis, Characterization, and Reactions of a Half-Metallocene Benzylidene Complex of Tantalum Bearing 2,3-Dimethyl-1,3-butadiene and Pentamethylcyclopentadienyl Ligands

Abstract

A half-metallocene benzylidene complex of tantalum, Ta(CHPh)Cp*(η 4 -supine-2,3-dimethyl-1,3-butadiene) (1), was prepared by treating TaCl 2 Cp*(η 4 -supine-2,3-dimethyl-1,3-butadiene) (2) with Mg(CH 2 Ph) 2 in toluene. The structure of 1 was determined by X-ray analysis, indicating the syn rotamer of the Ta=CHPh moiety; the Ph group points upward toward the Cp* ligand. The coordinatively unsaturated 16-electron complex 1 reacted with some unsaturated organic molecules (ethylene, cyclopentene, and tert-butyl cyanide), an alcohol, and an amine. The cycloaddition of ethylene and cyclopentene gave rise to metallacyclobutanes. The addition of tert-butyl cyanide to 1 afforded an imido tantalum complex, Ta(NC-( t Bu)=CHPh)Cp*(η 4 -supine-2,3-dimethyl-1,3-butadiene) (6), which was derived from the metathesis reaction of a nascent 2-aza-1-tantalacyclobut-2-ene intermediate. Protonolysis of 1 by addition of methanol and p-methoxyaniline gave a methoxy benzyl complex, Ta-(CH2Ph)(OCH3)Cp*(η 4 -supine-2,3-dimethyl-1,3-butadiene) (7), and an amido benzyl complex, Ta(CH2Ph)(NHC6H40CH3)Cp*(η 4 -supine-2,3-dimethyl-1,3-butadiene) (8), respectively. Upon heating at 60 °C, complex 8 was converted to an imido butenyl complex, Ta(CH 2 Ph)(NC 6 H 4 -OCH 3 )(η 1 -2,3-dimethyl-2-butene)Cp* (9). Structures of the amido complex 8 and the imido complexes 6 and 9 were determined by X-ray analysis.